Light emitting diodes (LEDs) are broadly used for light sources, displays, and signaling elements. As LEDs are more power efficient than conventional light sources, and since the packing density allows high quality displaying functionality, a significant increase in applications using LEDs can be observed. LEDs are typically used in string-like or array-like configurations, where a large number of light emitting diodes is coupled to form either stings or display panels, or to provide efficient light or backlight sources for numerous applications. Accordingly, there is a general motivation to provide power efficient, small, and cheap electronic devices for driving the LEDs. The conventional approach to drive LEDs consists in coupling a current source to the LEDs in order to provide a constant current through the LEDs, such that a specific intensity and color of the light emission is achieved. A more sophisticated conventional approach includes a switch, as for example a metal oxide silicon field effect transistor (MOSFET), which is coupled in series with the LEDs. The LED is switched on and off by the switch at a high frequency. The ratio of the ON- and OFF-periods allows to control the light emission of the LEDs. In addition to this well-known control mechanism, a variety of power management concepts is applied for the current sources or voltage sources. In order to provide a variety of different regulated output voltages and output currents from a single power source, the switched power regulators are used, such as boost-, buck-, and buck-boost-converters. Generally, LEDs are to be driven at a constant current. Switch-mode solutions are preferred, as they provide an improved efficiency for varying load conditions caused by production spread, temperature variations, and ageing of the LED forward voltage. Additionally, taking the whole system into consideration, low cost and good color stability are advantages of the switch mode solutions. The switch mode solutions are most appropriate for 0D and 1D dimming backlight systems for mass production. The basic principle of the switch mode power converters consists in supplying a specific current to an inductor (e.g. a coil), decoupling the voltage source from the inductor by a switch, and driving for a limited time a load by the energy stored in the decoupled inductor. Once a specific part of the energy in the inductor is spent, the inductor is again coupled to the voltage source. Particular arrangements of switches, inductors, diodes and capacitors in combination with specific switching mechanisms and sequences allow to provide output voltages in a wide range being above, below or above and below the input voltage.
Although switch mode power converters are beneficial in terms of power consumption and flexibility, a major drawback of the conventional solutions resides in the rather complex control mechanisms to establish well-defined conditions for the LEDs. Providing an appropriate current through the LEDs for a specific light emission and other parameters and preserving at the same time the suitable timing (e.g. for PWM) for the switched voltage or current sources impose high requirements on the control circuitry. If, for example, the control mechanism for the LEDs is too slow, variations of the input and output voltage, as well as variations of internal parameters will become visible in variations of luminance and color stability of the LEDs.